1. Field of the Invention
The present invention relates to a process flow for forming a solder layer including contact balls of a contact material, such as solder, for providing contact areas for directly attaching an appropriately formed package or carrier substrate to a die carrying an integrated circuit.
2. Description of the Related Art
In manufacturing integrated circuits, it is usually necessary to package a chip and provide leads and terminals for connecting the chip circuitry with the periphery. In some packaging techniques, chips, chip packages or other appropriate units may be connected by means of balls of solder or any other conductive material, formed from so-called solder bumps, that are formed on a corresponding layer, which will be referred to herein as a solder layer, of at least one of the units, for instance on a dielectric passivation layer of the microelectronic chip. In order to connect the microelectronic chip with the corresponding carrier, the surfaces of the two respective units to be connected, i.e., a microelectronic chip comprising, for instance, a plurality of integrated circuits and a corresponding package, have formed thereon adequate pad arrangements to electrically connect the two units after reflowing the solder balls provided at least on one of the units, for instance on the microelectronic chip. In other techniques, solder balls may have to be formed that are to be connected to corresponding wires, or the solder balls may be brought into contact with corresponding pad areas of another substrate acting as a heat sink. Consequently, it may be necessary to form a large number of solder balls that may be distributed over the entire chip area, thereby providing, for example, the I/O capability required for modern microelectronic chips that usually include complex circuitry, such as microprocessors, storage circuits and the like and/or include a plurality of integrated circuits forming a complete complex circuit system.
In order to provide hundreds or thousands of mechanically well-fastened solder balls on corresponding pads, the attachment procedure of the solder balls requires a careful design, since the entire device may be rendered useless upon failure of only one of the solder balls. For this reason, one or more carefully chosen layers are generally placed between the solder balls or the bumps, from which the solder balls are formed by reflowing, and the underlying substrate or wafer including the pad arrangement. In addition to the important role these interfacial layers, herein also referred to as underbump metallization layer, may play in endowing a sufficient mechanical adhesion of the bump to the underlying pad and the surrounding passivation material, the underbump metallization has to meet further requirements with respect to diffusion characteristics and current conductivity. Regarding the former issue, the underbump metallization layers have to provide an adequate diffusion barrier to prevent the solder material or bump material, frequently a mixture of lead (Pb) and tin (Sn), from attacking the chip's underlying metallization layers and thereby destroying or negatively affecting their functionality. Moreover, migration of bump material, such as lead, to other sensitive device areas, for instance into the dielectric, where a radioactive decay in the lead may also significantly affect the device performance, has to be effectively suppressed by the underbump metallization. Regarding current conductivity, the underbump metallization, which serves as an interconnect between the solder ball and the underlying metallization layer of the chip, has to exhibit a thickness and a specific resistance that does not inappropriately increase the overall resistance of the metallization pad/ball system. In addition, the underbump metallization will serve as a current distribution layer during electroplating of the bump material. Electroplating is presently the preferred deposition technique for solder material, since physical vapor deposition of solder bump material, which is also used in the art, requires a complex mask technology in order to avoid any misalignments due to thermal expansion of the mask while it is contacted by the hot metal vapors. Moreover, it is extremely difficult to remove the metal mask after completion of the deposition process without damaging the solder pads, particularly when large wafers are processed or the pitch between adjacent solder pads is small.
Although a mask is also used in the electroplating deposition method, this technique differs from the evaporation method in that the mask is created using photolithography to thereby avoid the above-identified problems caused by physical vapor deposition techniques. However, electroplating requires a continuous and highly uniform current distribution layer adhered to the substrate that is mainly insulative, except for the pads on which the bumps will be formed. Thus, the underbump metallization also has to meet strictly set constraints with respect to a uniform current distribution as any non-uniformities during the plating process may affect the final configuration of the bumps and, after reflowing, the bumps of the resulting solder balls in terms of, for instance, height non-uniformities, which may in turn translate into fluctuations of the finally obtained electric connections and the mechanical integrity thereof. The height of the bumps is, among others, determined by the local deposition rate during the electroplating process, which is per se a highly complex process, so that process non-uniformities resulting from irregularities of the plating tool or any components thereof may also directly cause corresponding non-uniformities during the final assembly process. The underbump metallization layer is patterned by means of appropriate etch techniques to provide well-defined islands below the solder material, thereby providing a well-defined wetting layer for the subsequent reflow process during which the solder bumps are shaped into spheres or balls. The size and thus the height of these solder balls is critical for the actual attachment of the chip to the carrier substrate, since any height variations may lead to a reduced contact in the final reflow process for connecting to the respective solder pad of the carrier substrate.
During reflowing, the solder material for forming the solder balls, especially any tin contained therein, may form an intermetallic phase with the copper of the uppermost sub-layer of the underbump metallization layer, thereby creating a reliable metallization interface. Moreover, during the reflow process, an oxide layer comprised of lead and tin forms on the surface of the solder ball and imparts a shiny appearance to the solder ball. The oxide layer acts as a passivation layer during subsequent manufacturing processes, such as substrate dicing and the like, wherein the integrity of the solder balls is to be maintained so as to substantially avoid any additional non-uniformities of the solder balls. Thus, the oxide layer desirably exhibits a high stability during the further assembling process, yet should be readily removable by a flux material prior to the final solder process for attachment to the carrier substrate. During the removal of the oxide layer, however, non-removed residuals of the oxide may significantly affect the solder process, thereby possibly causing a non-wet contact with the solder pad of the carrier substrate. Hence, a less reliable connection or a total failure of the connection may result. As previously explained, great efforts are made for improving the complex process related to the formation of the underbump metallization layer and the solder bumps; however, non-uniformities encountered during the reflow process may be of comparable importance.
In view of the above-described situation, a need exists for an enhanced technique that may avoid or at least reduce the effects of one or more of the problems identified above.